When loading an pipetter with liquid, to which stop do you depress the plunger?

Question

BlackTom AI · Accepted Answer

The first stop on an air-displacement pipette corresponds to the set-volume displacement of the piston, so depress to the first stop when loading (aspirating) liquid.

Relevant knowledge points and concepts:
- Air-displacement pipettes have two detents (stops): the first stop withdraws the piston to the exact volume set (used for aspirating and normal dispensing). The second stop (blow-out) pushes additional air to expel any residual liquid from the tip.
- Aspiration: displace air by depressing to first stop, immerse tip 2–3 mm below the meniscus, then release plunger slowly to draw the set volume into the tip.
- Dispensing: depress to the first stop to deliver the set volume; use the second stop only when performing a blow-out to clear residual liquid (if required by technique).

Step-by-step reasoning and procedure:
1. Set the pipette to the desired volume. Attach an appropriate disposable tip.
2. Depress the plunger smoothly to the first stop (this displaces a fixed volume of air equal to the set volume).
3. Immerse the tip correctly and release the plunger slowly and steadily; the displaced air creates a vacuum that draws the exact set volume of liquid into the tip.
4. Transfer to the receiving vessel and depress the plunger to the first stop to dispense the set volume. If protocol calls for blow-out, depress to the second stop after the first stop to expel residual liquid.
5. Release and eject tip.

Why the correct answer is correct:
- Depressing to the first stop before aspirating ensures you displace only the set volume of air; releasing draws exactly that volume of liquid into the tip. The second stop would push extra air and either prevent proper aspiration or cause inaccurate uptake.

Why other options are incorrect:
- b. “First stop, pause, then second stop” is incorrect for loading—using the second stop at this stage would force extra air out and prevent correct aspiration (second stop is for blow-out during dispensing).
- c. “Straight to second stop” is incorrect because the second stop displaces more air than the set volume and would produce incorrect aspiration and potential splashing.

Conclusion: Depress to the first stop only when loading a pipetter with liquid. This matches the provided correct answer: a. First stop only.

When loading an pipetter with liquid, to which stop do you depress the plunger?单项选择题

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